Electroluminescent (EL) panels provide an alternative to incandescent lighting in many applications, such as automotive or aircraft instrument panels. Uniform light generation without color shift, compact size, and low heat output make EL panels ideal for such applications.
There are, however, inherent characteristics of EL panels and conventional drive systems that limit their desirability in these applications. Most significantly, as a typical EL panel is driven at a constant voltage for a period of time, the light intensity output decreases, resulting in a short panel life span. Additionally, under constant drive conditions, as temperature changes, the panel intensity varies. Humidity and ambient temperature affect panel life. Also, the turn-on intensity is higher than the turn-off intensity. Present inverter-panel lighting systems must be designed to allow excessively high intensity levels at the beginning of lamp operation to guarantee sufficient intensity to the end of operation time target.
Previous inverter driver circuits have tried to compensate for intensity changes by using indirect methods. These methods utilize the decrease in panel capacitance, which occurs as the panel ages. The most common technique is to design the inverter-panel system to increase the drive voltage and frequency as the lamp capacitance decreases. This is achieved by designing the drive-panel system as a tuned LCR oscillator. However, this indirect method does not provide sufficient control for maintaining light output levels.